Event-processing systems continuously process events in real time as they are received. Events are data items that include a payload of data and an associated time. Event processing operates with respect to asynchronous event sequences or event streams, which are sequences of events that happen over time. Examples of event sequences include stock ticks identifying changes in price of a stock and social media messages. Processing is specified over an event sequence with a query to enable projection, filtering, and aggregation, for instance. Unlike traditional database queries, event-processing queries are not evaluated once over stored data. Rather, event-processing queries are evaluated continuously over events as the events occur. To enable substantially real time processing, event-processing systems store queries in memory and evaluate the queries as events arrive in the system.